Systemic juvenile idiopathic arthritis mimics multicentric Castleman's disease.

An 11-year-old girl presented with fever and a large cervical lymphadenopathy. Indicators of inflammation were remarkable: she had extremely high levels of serum interleukin-6 (IL-6) (398 pg/ml) in addition to hypergammaglobulinemia and hypoalbuminemia. Computed tomography (CT) revealed swollen systemic lymph nodes. Two weeks after the onset of symptoms she developed polyarthralgia. Biopsy of the cervical lymph node revealed massive infiltration of plasma cells without hyaline vascular changes. She was diagnosed with systemic juvenile idiopathic arthritis (JIA). The patient's symptoms and hypercytokinemia disappeared soon after corticosteroid treatment was started. This case demonstrates that overproduction of IL-6 is common to systemic JIA and multicentric Castleman's disease.